The court said in a 6-0 opinion that plaintiffs must show some relationship between the amount of exposure and development of the disease.

The ruling came in a 2005 lawsuit by a western Pennsylvania auto mechanic against Ford Motor Co., Allied Signal Inc. and others alleging that his exposure to asbestos through repair of brake linings over 44 years had caused mesothelioma. The justices said they took up the matter “as a test case for the admissibility of expert opinion evidence to the effect that each and every fiber of inhaled asbestos is a substantial contributing factor to any asbestos-related disease.’’

The defense argued that it would be impossible to legally assign responsibility without showing that mechanic Charles Simikian had been exposed to specific doses of asbestos known to cause the disease. Allegheny County Common Pleas Court Judge Robert Colville agreed, saying there was no credible explanation for a determination that the disease was due to workplace exposure to asbestos and not another source.

A Superior Court panel said the “any-exposure’’ testimony was something for a jury, not the trial court, to evaluate, but the high court said the judge “was right to be circumspect about the scientific methodology underlying the any-exposure opinion.’’

“He spent considerable time listening to the attorneys’ arguments but was unable to discern a coherent methodology supporting the notion that every single fiber from among, potentially, millions is substantially causative of disease,’’ Saylor’s decision said. “Moreover, he appreciated the considerable tension between the any-exposure opinion and the axiom (manifested in myriad ways both in science and daily human experience) that the dose makes the poison.’’

The justices said even the analogies used by the plaintiff’s expert witness were “fundamentally inconsistent with both science and the governing standard for legal causation.’’ For example, they said, in making an analogy to cigarette use he “offered no scientific basis for concluding that a single cigarette of the potentially halfmillion a person might smoke in a lifetime is substantially causative of such person’s lung cancer.’’

The high court justices sent the case back to the lower court to consider whether there were other appeal issues.

Attorneys for the plaintiff did not return a phone call seeking comment, The Philadelphia Inquirer said.

The National Federation of Independent Business, which says thousands of businesses around the country had been sued on the basis of the theory, hailed the decision.

Bill Anderson, a lawyer with Crowell & Mooring who filed a friend-of-the-court brief on behalf of the Pennsylvania Chamber of Commerce, said only one other state high court – that of Texas – had barred use of the any-fiber theory of causation increasingly used in asbestos cases across the country.